Through the study of acquired and inherited hypophosphatemic disorders a new understanding of the hormonal regulators of phosphate homeostasis is emerging. FGF-23 plays a central role in the pathophysiology of several forms of hypophosphatemic rickets including tumor-induced osteomalacia (TIO), X-linked hypophosphatemic rickets (XLH) and autsomal dominant hypophosphatemic rickets (ADHR). However, the mechanism of by which this potent inhibitor of proximal renal tubular phosphate transport modulates the main hormone-responsive sodium-dependent phosphate transporter (NaPi-lla) is virtually unexplored. We hypothesize that FGF-23 inhibits sodium-dependent phosphate transport by promoting internalization of NaPi-lla via activation of FGF receptors. Furthermore, internalization is accomplished by the assembly of a macromolecular complex involving NaPi-lla and signaling molecules on the scaffold of Sodium hydrogen ion exchanger related factor-1 (NHERF-1) that interacts with the actin cytoskeleton. In Specific Aim 1, we will determine if FGF-23 acts via known FGFRs to promote NaPi-lla internalization. We will further define the major signaling pathway(s) responsible for NaPi-lla trafficking. In Specific Aim 2, we will examine role of NHERF-1 in FGF-23 mediated internalization of NaPi-lla. In Specific Aim 3, we will identify NHERF-1 binding partners which serve to integrate FGF-23 signaling and NaPi-lla internalization. The experiments outlined in this proposal will establish the mechanism by which FGF-23 regulates phosphate handling in the proximal renal tubule including control of NaPi-lla trafficking, activation of the cell signaling pathways and the assembly of NHERF-1 dependent multiprotein complexes that integrate signaling and NaPi-lla membrane localization. A better understanding of FGF-23 signaling and its consequences for renal phosphate handling and ultimately skeletal mineralization will provide new therapeutic targets for disorders of phosphate homeostasis.